Distributed optical chemical sensor

ABSTRACT

The invention relates to a sensor system comprising a waveguide, which waveguide comprises a grating in at least a part of the waveguide, which waveguide further comprises a coating, the coating comprising a polymer, which polymer comprises a chain, in which chain are present an aromatic group and a chemical group selected from the group of sulfonyl groups, carbonyl groups, carbonate groups, fluorocarbon groups, siloxane groups, pyridine groups and amide groups.

The invention relates to a sensor system, to a waveguide comprising agrating and a sensor material, to the sensor material, to a method ofpreparing the waveguide, to a sensor system comprising the waveguide,and to the use of the sensor system for measuring an environmentaleffect.

Optical sensors are sensors of which the sensing principle andoptionally the data transfer make use of electromagnetic radiation.Accordingly, optical sensors have a number of advantages over electronicdetection systems. Optical sensors are for example more reliable inenvironments that are difficult to access and/or hazardous to humans,environments such as those found in the oil and gas industry, and areusually not adversely affected by the electromagnetic radiation that isgenerally produced in for example power cable systems, inductionfurnaces or equipment for nuclear magnetic resonance measurements, suchas MRI or NMR equipment. Other advantages are the easy operation ofoptical sensors on large distances, their small size, their flexibilityand/or the possibility to make a sensor system comprising an array ofdiscrete sensors that all may be read separately from a single opticalfibre (a multiplexed sensor system).

Typical sensors that are based on waveguide grating are, e.g., describedin detail in U.S. Pat. No. 5,380,995, U.S. Pat. No. 5,402,231, U.S. Pat.No. 5,592,965, U.S. Pat. No. 5,841,131, U.S. Pat. No. 6,144,026, US2005/0105841, U.S. Pat. No. 7,038,190, US 2003/156287.

One principle on which such sensors may be based is an axial strain ofthe waveguide, as a result of an environmental effect that is to bedetected, for example by using a coating on the waveguide that deformsunder the influence of the environmental effect. An important method viawhich (a change in) axial strain of the waveguide becomes detectable isto use a grating in the waveguide. When such a grating, guiding aspecific spectrum of electromagnetic radiation, stretches or shrinksunder the axial strain, the spectral pattern of transmitted radiationand/or the spectral pattern of reflected radiation (i.e. the spectralresponse) changes. Such changes in the spectral response provide—whenmeasured—quantitative information on the environmental effect. Twoexamples of a grating are a Fibre Bragg Grating (FBG) and a Long PeriodGrating (LPG). A multiplexed sensor system can be prepared using aplurality of gratings, in particular FBG's.

For example, US application 2005/0105841 relates to the use of apolyethyleneimine (PEI) monolayer coating on a Long Period Gratingwaveguide. The coating swells under the uptake of water, which makes awaveguide comprising such coating suitable for measuring relativehumidity (RH), based on changes of the refractive index of the coating.

Although it has been widely recognized that optical sensors have anumber of advantages over electronic measuring systems, the fullpotential of optical sensors has not yet been realized. In particular,there is a need for improved sensors for use under extreme conditions,for example under high pressure and/or high temperature. Examples ofextreme conditions are conditions that may exist in underground oil orgas reservoirs, or in the equipment that is used to produce oil or gasfrom these reservoirs. It would in particular be desired to provide animproved sensor for the detection of compounds such as methane, carbondioxide, hydrogen sulfide or water, e.g. under the conditions asmentioned above. A compound that is to be detected may hereinafter bereferred to as ‘analyte’.

WO 03/056313 describes an optical sensor that can operate in offshoreenvironments. The sensor relies on the principle that hydrogen candiffuse into an optical fiber, which results in transmission loss thoughthe fibre at specific wavelengths. The quantity of such a loss is ameasure for the amount of hydrogen present in the environment. Alimitation of this sensor is that the loss of transmission is generallypermanent, because the in-diffused hydrogen causes irreversible changesin the fiber.

It is an object of the present invention to provide a novel sensorsystem.

It is also an object of the present invention to provide a novelwaveguide comprising a grating, which waveguide can be used in anoptical sensor, in particular a sensor with a detection mechanism thatis based on an FBG or an LPG, that can serve as an alternative to knownwaveguides comprising a grating.

It is in particular an object of the present invention to provide anovel method for preparing a waveguide according to the invention, inparticular a method that allows the manufacturing of a multiplexedsensor system in an industrially attractive manner.

It is in particular an object of the invention to provide a novelwaveguide that is suitable for use under extreme conditions, such asunder conditions that may exist in underground oil or gas reservoirs, orin the equipment that is used to produce oil or gas from thesereservoirs.

It is a further object of the invention to provide a novel waveguidethat is improved, in particular in that a detection system comprisingthe waveguide is improved in that it offers at least one of thefollowing advantages: a higher selectivity towards a specificenvironmental effect, a larger dynamic range, a higher accuracy, ahigher robustness, a lower detection limit and a higher sensitivity.

The selectivity of a detection system for measuring a certainenvironmental condition is the extent to which the detector specificallyreacts to a change in a selected environmental condition, without beingaffected by a change in other conditions.

The dynamic range of a sensor system is the range of a changeablequantity that can be measured with that sensor system, the limits ofwhich range are defined by the smallest and the largest value of thechangeable quantity that can be measured with that sensor system.

The accuracy of a detection system is the closeness of a reading orindication of that detection system to the actual value of the quantitybeing measured.

Robustness is the extent to which a detection system is resistant tochanges in the detection system, influences from a specific sample andinfluences from the environment other than the condition, other than thechanges in the condition to be measured. Accordingly, as a system ismore stable, the back ground noise will be less and/or fewer artefactswill occur in the measuring signal, such a spikes, base line driftand/or base line shifts.

The detection limit is the lowest measurable change in a environmentalcondition. It is determined by the signal to noise ratio. In general,the detection limit for a particular substance is set at a signal tonoise ratio of 2 (if the noise is represented as peak to peak) or 4 (ifthe noise is represented as the root of the mean square noise (RMSnoise)).

The sensitivity of a detection system is the smallest change in aenvironmental condition, such as a physical or chemical parameter, thatcan be detected by the detection system.

It has now been found that one or more of these objects are realised byproviding a waveguide having a coating which comprises a polymercomprising specific groups.

Accordingly, the invention relates to a sensor system for detecting achemical substance (analyte), comprising a waveguide, which waveguidecomprises a grating in at least a part of the waveguide, which waveguidefurther comprises a coating, the coating comprising a polymer, whichpolymer comprises a chain having an aromatic group and a furtherchemical group selected from the group of sulfonyl groups, carbonylgroups, carbonate groups, fluoro carbon groups, siloxane groups,pyridine groups and amide groups.

Such a sensor system may in a preferred embodiment be used under extremeconditions, such as under conditions that may exist in underground oilor gas reservoirs, or in the equipment that is used to produce oil orgas from these reservoirs.

In a preferred embodiment, the chain further comprises imide groups. Inanother preferred embodiment, these imide groups may also be present inthe polymeric coating in a different chain, such that the polymericcoating is a blend of two or more polymers. In a specific embodiment,the chain further comprises oxygen atoms. The presence of sulfonylgroups, carbonyl groups, carbonate groups, amide groups, fluoro carbongroups, siloxane groups, pyridine groups, imide groups or oxygen atomsin the chain is considered beneficial for improved interaction with ananalyte of interest, thereby improving, e.g. the sensitivity and/or thedynamic range.

Preferably, a sensor system according to the invention comprises asource for providing electromagnetic radiation and a photo-detector.

The invention further relates to a waveguide, comprising a grating in atleast a part of the waveguide, which waveguide comprises a coating, thecoating comprising a polymer, which polymer comprises a chain having anaromatic group and a chemical group selected from the group of sulfonylgroups, carbonyl groups, carbonate groups, fluoro carbon groups,siloxane groups, pyridine groups and amide groups. In a preferredembodiment, the chain further comprises imide groups. In a specificembodiment, the chain further comprises oxygen atoms.

In the present disclosure, the term waveguide is used for opticalwaveguides. An optical waveguide is a physical structure that guideselectromagnetic waves in at least part of the optical spectrum, i.e. inat least part of the spectrum formed by the infrared, visible andultraviolet ranges of the electromagnetic spectrum.

Usually, a waveguide is of elongate form. In general, a waveguide iscylindrical, in particular with a circular cross-section. A waveguidegenerally comprises an assembly of a core and a cladding covering thecore. The core as well as the cladding usually have a substantiallycircular cross-section. The center of the cross-section of the claddingusually coincides with the center of the cross-section of the core (FIG.1). As discussed below, in specific embodiments the cross-section of thecore and/or the cladding may be different. With a cross-section of thewaveguide is meant a section through the waveguide that is a plane thatis perpendicular to the longitudinal direction of the waveguide.

Common types of waveguides include optical fibres, e.g. as referred toin the above cited prior art, and rectangular waveguides. Waveguides arecommercially obtainable from various sources. Manufacturing andapplications can be found in the Encyclopedia of Laser Physics andTechnology (http://www.rp-photonics.com/encyclopedia.html). Fibre BraggGratings are supplied by FOS&S, Geel, Belgium.

In the present disclosure, with “grating” is meant a periodic variationof the refractive index of waveguide material in a segment of awaveguide core. A grating reflects particular wavelengths ofelectromagnetic waves and transmits other wavelengths, and can be usedas an inline optical filter or as a wavelength-specific reflector. Agrating in a waveguide according to the invention may in particular be aFibre Bragg Grating (FBG).

As indicated above, the coating of the waveguide comprises a polymer; apolymer is a substance of which the molecules, in particular organicmolecules, are built up from a plurality of monomeric units. The polymerof the coating is usually built up from at least 10 monomeric units,preferably at least 50 monomeric units, at least 100 monomeric units, orat least 250 monomeric units. The upper limit of the polymer is notparticularly critical and can be, for instance, 1 000, 10 000, 50 000,or more than 50 000 monomeric units. The monomeric units may be the same(a homopolymer) or the polymer may be composed of two or more differentmonomers (a copolymer).

The polymer of the coating may be branched or linear. The polymer may becross-linked or uncrosslinked. In case the polymer comprises more thanone chain per polymer molecule, typically at least the main-chain has anaromatic group and a group selected from the group of sulfonyl groups,carbonyl groups, carbonate groups, imide groups, fluoro carbon groups,siloxane groups, pyridine groups and amide groups. Side-chains may alsocomprise such groups. In case the polymer is such that it contains aplurality of (major) chains without one chain being the main chain (asmay be the case in e.g. hyperbranched polymers) preferably most or allof said chains have an aromatic group and a group selected from thegroup of sulfonyl groups, carbonyl groups, carbonate groups, imidegroups, fluoro carbon groups, siloxane groups, pyridine groups and amidegroups. Thus, it should be understood that said groups form part of thebackbone of the polymer and the polymer is thus distinguishable from apolymer wherein an aromatic group and a group selected from the group ofsulfonyl groups, carbonyl groups, carbonate groups, imide groups, fluorocarbon groups, siloxane groups, pyridine groups and amide groups arependant.

A preferred aromatic group in a chain of said polymer is a phenyl group,preferably a p-phenylene group, which may comprise substituents. Otherpreferred aromatic groups are selected from the group of naphthalenegroups.

In a preferred embodiment, the sulfonyl groups, carbonyl groups,carbonate groups, imide groups, siloxane groups, pyridine groupsrespectively amide groups are directly attached to the aromatic group.Thus, a preferred polymer molecule may comprise the following structure:—[Ar—X—]_(n). Herein ‘n’ is an integer representing the number ofmonomeric units. ‘Ar’ represents the aromatic group, each Xindependently comprises a group selected from sulfonyl groups, carbonylgroups, carbonate groups, imide groups, siloxane groups, pyridine groupsand amide groups, with the proviso that at least one of the X'srepresents a sulfonyl group, a carbonyl group, a carbonate group, asiloxane group, a pyridine group or an amide group.

In a further embodiment, at least one X represents a organofluorinegroup. Organofluorine groups (also known as fluoro carbons) are groupscomprising carbon, fluorine and optionally one or more other groups, inparticular one or more hydrogen atoms. In particular, the organofluorinemay be represented by the formula-C_(m)F_(k)H_(l)—, wherein m is aninteger, e.g. in the range of 1-10, in particular in the range of 2-6.As generally known in chemistry ,the values for k and l depend on thevalue for m and the number of unsaturated carbon carbon bonds. Theinteger k is in the range of 1 to 2m, the integer l is in the range of 0to 2m-1, with the proviso that the sum of k and l is 2m (if nounsaturated bonds are present) or less (if one or more unsaturated bondsare present). In particular the —C_(m)F_(k)H_(l)— group may be ahydrofluoroalkyl or a perfluoroalkyl. In case of a hydrofluoroalkyl thesum of k+l equals 2m and k and l are both at least 1. In case of aperfluoroalkyl k equals 2m and l is 0. A preferred perfluoroalkyl ishexafluoroisopropyl. The number of fluorine atoms in a organofluorine ispreferably equal to or higher than the number of hydrogen atoms, forimproved interaction with an analyte. Two aromatic groups in the polymerchain can also be separated by an oxygen molecule. Thus, a preferredpolymer may comprise the following structure: —[Ar—O—Ar—X—]_(n), whereinX and n are as identified above. Two aromatic groups in the polymerchain can also be separated by an analyte specific group (i.e. groupcapable of selectively interacting with an analyte of interest, therebycausing a change in the polymer material), such as hexafluoroisopropyl,another alkyl comprising fluorine groups, or isopropyl, siloxane orpyridine. Thus, a preferred polymer may comprise the followingstructure: —[Ar—C(CF₃)₂—Ar—X—]_(n), wherein X and n are as identifiedabove.

In a preferred embodiment, the polymer is selected from the group ofpolysulfones comprising aromatic groups in the chain and polycarbonatescomprising aromatic groups in the chain. Any of these may in particularbe used for a waveguide of a detection system for detecting H₂S. In aspecific embodiment, the polymer also comprises imide groups in thechain or the polymer is a blend of a polymer comprising at least onepolymer selected from the group of polysulfones comprising aromaticgroups in the chain and polycarbonates comprising aromatic groups in thechain and further comprising a polymer comprising imide groups andaromatic groups in the chain.

Such an embodiment is in particular preferred for a high sensitivityand/or a high temperature resistance.

In particular, the polysulfone may be selected from the group ofpoly(diphenyl sulfones). Preferred polysulfones arepoly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylidene-1,4-phenylene)and poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene).

In particular, the polycarbonate may be selected from the group ofpoly(diphenyl carbonates). Preferred polycarbonates arepoly(oxycarbonyloxy-1,4-phenyleneisopropylidene-1,4-phenylene) andpoly(oxycarbonyloxy-1,4-phenylenehexafluoroisopropylidene-1,4-phenylene).

In particular, the polyimide may be selected from the group of aromaticfluorocarbon polyimided. A preferred polyimide ispoly(4,4′-(sulfonylbis(4,1-phenyleneoxy))dianiline-co-4,4′-(hexafluoro-isopropylidene)diphthalic anhydride).

A preferred polyamide is poly(trimellitic anhydridechloride-co-4,4′-diaminodiphenylsulfone).

The silixone may in particular be a dialkylsiloxane, which alkyl maycomprise one or more substituents, e.g. on ore more fluorine atoms. Apreferred siloxane is dimethylsiloxane. The siloxane may advantageouslybe present in a detection system for CO₂.

A preferred polysiloxane polymer in a coating of a sensor system of theinvention is polysiloxane, preferably poly (1,3-bis(3-aminopropyl)tetramethyldisiloxane-co-4,4′-(hexafluoro-isopropylidene) diphthalicanhydride).

For H₂S detection, poly(4,4′-(sulfonylbis(4,1-phenyleneoxy))dianiline-co-4,4′-(hexafluoro-isopropylidene) diphthalic anhydride) andpoly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylidene-1,4-phenylene)are particularly suitable.

If desired, the selectivity of the coating for a specific analyte may beenhanced by including one or more functional groups that are capable ofspecifically interacting with the analyte to be detected. Suchfunctional group having affinity for a specific analyte may be includedin the chain, or be pendant from the chain. For example, the polymer maycomprise amine side-chains, in particular for an increase in interactionwith hydrogen sulfide. The polymer may comprise halogenated alkylmoieties, e.g. hexafluoro propyl groups, which may in particular bepresent in the chain. Such groups increase interaction with polaranalytes and may in particular increase the dynamic range and/or thesensitivity.

The polymer of the coating may comprise cross-links. If cross-links arepresent, a typical cross-linking degree is 1 to 50 cross-links per 100monomer units. The polymeric chains may be cross-linked by reacting thepolymer with a cross-linker (i.e. a compound capable of initiating thecross-link reaction or a multi-functional monomer, for example 1 to 30wt % of crosslinker, based on the total weight of the polymer beforecrosslinking.

Several cross-linkers are known in the art. Preferred examples ofcrosslinkers are polyfunctional peroxides.

It is also possible to prepare a crosslinked polymer by polymerising amonomer mixture comprising at least one monomer for forming thealiphatic chain and at least one multifunctional monomer for forming thecrosslinks. The concentration of multifunctional monomer may forinstance be chosen in the range of 1 to 30 wt % based on total monomers.In the case of polyimides, preferred examples of crosslinkers arepolyfunctional anhydrides or amines.

In an advantageous embodiment, the presence of aromatic group(s) andsaid further chemical group(s) in a polymer according to the inventionresults in a high stiffness and high temperature resistance.Accordingly, a coated waveguide according to the invention is suitablefor use under extreme conditions such as a high pressure and/or a hightemperature. In particular, in an embodiment wherein the chain of thepolymer comprises imide groups, the polymer has a particularly hightemperature resistance.

The polymer coating of the present invention is usually resistant totemperatures higher than 100° C., preferably higher than 150° C., inparticular higher than 180° C. Resistance against high temperaturesmeans that the softening temperature or glass transition temperature(T_(g)) is higher than the temperatures given above. In particular,resistance against high temperatures means that the temperature at whichchemical degradation of the coating occurs is higher than thetemperatures given above. For a high temperature resistance, inparticular a polymer selected from the group consisting of polysulfones(polyethersulfones, polyphenylsulfones), polycarbonates, polyketones,polyimides, polysiloxanes and polyamides is suitable.

A sensor system according to the invention may advantageously be usedunder extreme conditions, such as under conditions that may exist inunderground oil or gas reservoirs, or in the equipment that is used toproduce oil or gas from these reservoirs. Such extreme conditionsinclude high temperature (which may be over 50° C., over 70° C. or evenover 100° C., e.g. up to 250° C., up to 200° C. or up to 150° C.,depending on the depth), high pressure (which may be over 50 bar, over100 bar, e.g. up to 200 bar or up to 150 bar, depending on the depth)and/or high salt concentration (e.g. saturation or oversaturation withNaCl).

Preferably, polymers according to the invention are amorphous, or atleast generally substantially amorphous. The T_(g) of the polymer isusually 100° C. or more, preferably 150° C. or more, more preferably180° C. or more.

The T_(g) as used herein is the T_(g) as determined from the secondheating curve obtained by Differential Scanning calorimetry (DSC) usinga heating rate and a cooling rate of 10° C./min (10 mg of sample under anitrogen atmosphere).

A chain of a polymer according to the invention may comprise oxygenatoms. The presence of oxygen in the chain, in particular as an etherunit, provides a higher sensitivity and/or a higher dynamic range of thesensor system.

A coating layer according to the invention usually has a thickness of atleast 0.5 μm, preferably of at least 10 μm, more preferably of at least20 μm. Usually, the thickness is 200 μm or less, in particular, at most100 μm, preferably 75 μm or less, more preferably 50 μm or less. Arelatively thin layer is advantageous for a short response time, arelatively thick layer is advantageous for a high sensitivity.

In a specific embodiment, a waveguide according to the invention has abarrier layer that is impermeable to hydrogen (gas). The permeability isfor example less than 10¹⁵ molecules/s.cm.atm^(1/2). Such layer ispreferably present between the cladding of the waveguide and thepolymeric coating, and may protect the waveguide from harmful effects ofthe hydrogen. For example, such a layer may comprise a material selectedfrom the group of carbon, silicon carbide, silicon nitride and metals.

The invention is advantageous in that it is possible to provide acoating that can reversibly absorb an analyte of interest in order toperform a continuous measurement of the presence of the analyte. With acontinuous measurement is meant a measurement in a non-cumulative way.For example, in a continuous measurement it is possible to measurefluctuations of an environmental effect, such as fluctuations in theconcentration of a certain chemical. This is in contrast to a cumulativeway of measuring, wherein the total amount of the chemical is observed(like in a dosimeter), i.e. only one or more increases can be observed.

The ability of a sensor system according to the invention to perform acontinuous measurement is demonstrated in FIG. 7 (see also Example 1).Herein is shown, that after the exposure to a certain analyte had beenstopped, the spectral response (i.e. the shift in the Bragg reflectionwavelength) tended to revert back to the pattern before the exposure ofthe polymer to the certain analyte started.

In the field of oil exploration and in the field of gas exploration, itis highly preferred to monitor the downhole environment for a longperiod of time without replacing the sensor system. In suchapplications, it is advantageous to use a sensor system according to theinvention, because such a system can perform continuous measurements andhas a high resistance against the conditions that may be present indownhole environments such as oil wells or gas wells.

A sensor system according to the invention may in particular be used fordetecting in an environment at least one analyte selected from the groupof alkanes (in particular methane, ethane, or propane), carbon dioxide,hydrogen, hydrogen sulphide, water, carbon monoxide, oxygen, hydrogencyanide and ammonia, in particular hydrogen sulphide and carbon dioxide.

In a specific embodiment, a (multiplexed) sensor system according to theinvention is used for detecting a water-oil or water-gas interface, formonitoring the displacement of such interfaces or for monitoring theconditions in the proximity of such interfaces. This can be done byusing a waveguide of which the coating is capable of interaction with acomponent of the water phase (e.g. water, NaCl) or a component of theoil/gas phase (e.g. H₂S, CH₄). The interface may in particular bemonitored in (underground) an oil/gas reservoir.

A sensor of the invention is in particular suitable for detecting agaseous or vaporous analyte.

In a specific embodiment, a waveguide according to the inventioncomprises a plurality of gratings, which are typically spatially apart,preferably 2-500, in particular 2-100 gratings. A waveguide having aplurality of gratings may be used in a multiplex detection system,wherein the spatially apart grating may be provided with a coatingcapable of interacting with the environmental effect, such as thepresence of an analyte of which the presence is to be detected Inparticular for a fibre Bragg grating, it is useful to have a pluralityof gratings. This allows each grating on the waveguide to be designed insuch a way that it creates a spectral response that is unique withrespect to the other gratings on the waveguide. This allows, forinstance, a single waveguide to be used to measure an environmentaleffect at a plurality of places. From a change in a specific uniquespectral response (measured at one or both of the ends of a waveguide)it will be clear in the vicinity of which grating an environmentaleffect has changed. In particular in case different gratings are coatedwith different polymeric materials, adapted to respond towards a changein different environmental effect, this also allows the use of a singlewaveguide to measure a multitude of environmental effects.

In a specific embodiment, a waveguide according to the inventioncomprises a multitude of gratings, at least part of which are present aspairs. If desired, the gratings of a pair can be spatially apart. Afirst grating of each pair may be used to measure (a change) in acertain environmental effect, and a second grating of each pair isuncoated or is coated with a coating that is insensitive, or at leastnot sensitive to a measurable extent, to the environmental effect to bemeasured with first grating of the pair. The second grating may be usedfor monitoring the temperature, and may in particular be used to correctfor the influence of temperature on the first grating of the pair.

In a further embodiment, a grating is partly coated (in the longitudinaldirection of the waveguide) with a coating that is sensitive to ameasurable extent to the environmental effect to be measured. Thegrating is e.g. coated for only about half of its length. Accordingly,the coated part of the grating then in essence forms the first gratingand the part of the grating that is not coated with the sensitivecoating forms the second grating.

Usually, a waveguide according to the invention comprises an assembly ofa core and a cladding. The electromagnetic radiation used for measuringpredominantly propagates through the core. The cladding usually enclosesthe core; it may protect the core, and/or aid in the propagation ofradiation through the core.

In a preferred embodiment, the coating comprises particles. Inparticular, the particles may be embedded in the polymer comprising achain, in which chain are present an aromatic group and a chemical groupselected from the group of sulfonyl groups, carbonyl groups, carbonategroups, fluoro carbon groups, siloxane groups, pyridine groups, andamide groups.

As used herein, particles include particles which are typically composedof solid or semi-solid materials. Typically, the (weight) averagediameter of such particles ranges from approximately 10 nm toapproximately 10 μm. A preferred average diameter is in the range of50-5000 nm, in particular in the range of 50-1000 nm. The averageparticle diameters are determinable by scanning electron microscopy(SEM). Particles having a average diameter of less than 500 nm arereferred to herein as nanoparticles. As will be understood by theskilled person, the size of the particles usually is equal to or lessthan the thickness of the coating.

In a preferred embodiment, a coating of a waveguide includes particles,in particular nanoparticles, that are capable of absorbing an analyte ofinterest (i.e. absorbent particles). It is contemplated that theparticles swell upon absorption, which results in deformation of thecoating, in an increase of axial strain in the waveguide, and ultimatelyin a change in the spectral response of the electromagnetic radiationthat is sent through the waveguide.

Further, in an advantageous embodiment, the particles are elastomericparticles. Typically, such (nano)particles that are capable of absorbingan analyte of interest are made of a material that has a low stiffness(e.g. E-modulus<100 MPa) and/or a low glass transition temperature (e.g.T_(g)<50° C.) (compared to the polymer comprising a chain, in whichchain are present an aromatic group and a chemical group selected fromthe group of sulfonyl groups, carbonyl groups, carbonate groups, fluorocarbon groups, siloxane groups, pyridine groups, and amide groups) andis not suitable for the same purpose when the particles are not embeddedin the polymeric coating.

In an advantageous embodiment, selectivity is enhanced by theintroduction of (polymer) (nano)particles that are capable ofselectively absorbing the analyte. In this case, a high extent ofabsorption of the analyte in the nanoparticles is combined with a highdiffusion speed (mobility) of the analyte in the coating polymer.

In a particularly advantageous embodiment the particles, which may benanoparticles, comprise a copolymer of a polyether and polyamide (e.g.Pebax polymers, for instance as available from Arkema) or a fluorocarboncomposition (e.g. fluoroalkyl(meth)acrylates, PTFE, FEP, PFA, MFA,etc.). Such particles may in particular be suitable for use in a coatingof a waveguide for use in the detection of H₂S.

In a further advantageous embodiment, said coating of the waveguidecomprises particles, preferably nanoparticles, selected from the groupof metal-organic frameworks (MOF's) particles. MOF's, also called“hybrid crystallised solids”, are coordination compounds with a hybridinorganic-organic framework comprising metal ions or semi-metal ions andorganic ligands coordinated to the metal ions. These materials areorganised as mono-, bi- or tri- dimensional networks wherein the metalclusters are linked to each other by spacer ligands in a periodic way.These materials generally have a crystalline structure and are usuallyporous. MOF's are in particular suitable for their good adsorptionproperties with respect to a gaseous analyte, for instance H₂, ahydrocarbon gas (such as CH₄) or CO₂.

The metal or semi-metal ions generally have a valence of at least +2.Common ligands include the conjugated bases of organic acids, such asbidentate carboxylates (e.g. oxalate, malonate, succinate, glutarate,phtalate, isophtalate, terephtalate), tridentate carboxylates (e.g.citrate, trimesate).

In a specific embodiment, the MOF is represented by the formulaM_(n)O_(k)X_(i)L_(p), wherein

-   each M is independently selected from the group of metal and    semi-metal ions, in particular selected from the group consisting of    Ti⁴⁺, Zr⁴⁺, Mn⁴⁺, Si⁴⁺, Al³⁺, Cr³⁺, V³⁺, Ga³⁺, In³⁺, Mn³⁺, Mn²⁺,    Mg²⁺ and combinations thereof;-   m is 1 , 2, 3 or 4, preferably 1 or 3 ;-   k is 0, 1 , 2, 3 or 4, preferably 0 or 1 ;-   i is 0, 1 , 2, 3 or 4, preferably 0 or 1 ;-   p is 1 , 2, 3 or 4, preferably 1 or 3 ;-   O is oxygen-   each X is independently selected from the group of anions, in    particular from the group of monovalent anions, more in particular    from the group consisting of OH⁻ Cl⁻, F⁻, I⁻, Br⁻, SO₄ ²⁻, NO³⁻,    ClO⁴⁻ PF⁶⁻, BF³⁻, —(COO)_(n−), R¹—(SO₃)_(n−), R¹—PO₃)_(n−), wherein    R¹ is selected from the group consisting of hydrogen and    hydrocarbons, in particular hydrogen and C1-C12 hydrocarbons, more    in particular hydrogen and C1-C12 alkyls, and wherein n is 1 , 2, 3    or 4;-   L is a spacer ligand, in particular a spacer ligand comprising a    radical R comprising q carboxylate groups (—COO⁻), wherein, q is 1 ,    2, 3, 4, 5 or 6, preferably 2, 3 or 4 . R may in particular be    selected from the group consisting of C1-C12 alkyl, C2-C12 alkene,    C2-C12 alkyne, mono- and poly-cyclic C6-C50 aryl, mono- and    poly-cyclic C3-C50 heteroaryl and organic radicals comprising a    metal material selected from the group consisting of ferrocene,    porphyrin, phthalocyanine and Schiff base R^(X1)R^(X2)—C═N—R^(X3),    wherein R^(X1) and R^(X2) are independently selected from the group    consisting of hydrogen, C1-C12 alkyl, C2-C12 alkene, C2-C12alkyne    and mono- and poly-cyclic C6-C50aryl and wherein RX3 is selected    from the group consisting of C1-C12 alkyl, C2-C12 alkene, C2-C12    alkyne and mono- and poly-cyclic C6-C50 aryl.

Such MOF's have been described in WO 2009/130251 of which the contentsare incorporated by reference, in particular with respect to thedefinitions M_(n)O_(k)X_(i)L_(p), at page 2 line to page 5, line 19.These MOF's may in particular be used for a sensor for detecting asulphur containing compound.

The amount of particles in the coating, is usually in the range of0.1-10 vol %, preferably in the range of 1-5 vol %.

The invention further relates to a blend material, comprising

-   a polymer comprising a chain having an aromatic group and a group    selected from the group of sulfonyl groups, carbonyl groups,    carbonate groups, imide groups, fluorocarbon groups, siloxane    groups, pyridine groups and amide groups; and-   particles composed of material that is capable of absorbing the    analyte.

The blend material may in particular be present as coating or partthereof on a waveguide, more in particular a waveguide of a sensorsystem as described herein. In a particular embodiment, the particlesare elastomeric nanoparticles and/or MOF nanoparticles.

With the term ‘blend material’, is meant a material that is a blend of apolymeric material and particles. With this term is also meant to beunderstood a material wherein the particles are bonded chemically withthe polymer molecules.

Preferred embodiments for the blend material are as described above forthe polymers/particles.

The invention further relates to the use of a sensor system fordetecting in an environment at least one analyte selected from the groupof alkanes (such as methane, ethane, or propane), carbon dioxide,hydrogen sulfide, hydrogen, water, carbon monoxide, oxygen, hydrogencyanide and ammonia.

The invention further relates to a method for preparing a waveguide, inparticular a waveguide according to the invention, comprising

-   providing a waveguide,-   preferably pretreating the waveguide with a silanisation agent,-   applying a mixture, comprising a solvent and a coating composition    comprising polymer or a precursor thereof, to at least part of the    surface of the waveguide, and-   curing the coating composition, whereby the solvent is at least    substantially evaporated or the precursor is reacted.

In an embodiment, the mixture that is applied comprises elastomericnanoparticles.

In an advantageous method of the invention, the waveguide or at least apart thereof to be coated is placed in a mould, leaving a space betweenthe outer surface of the waveguide or part thereof inside the mould andthe inner surface of the mould, introducing the coating composition intothe space; and curing the coating composition.

It is possible to coat a selected part of the waveguide. Such part isnot limited to an extremity of the waveguide. One or more parts remotefrom the extremities can be selectively coated.

By using a coating composition already comprising the polymer a coatingcan be provided without needing to include curing agents, initiators andthe like, although this is in principle possible, in particular in casethe polymer in the coating should be crosslinked.

The present application further is directed to a waveguide, whichwaveguide comprises a core and a cladding at least partially coveringthe core, wherein, in a cross-sectional plane that is perpendicular tothe longitudinal direction of the waveguide, the thickness of thecladding in a first radial direction in said plane is different from thethickness of the cladding in a second radial direction in said plane andto a sensor system comprising such a waveguide.

Such a sensor system may also be used under extreme conditions, such asunder conditions that may exist in underground oil or gas reservoirs, orin the equipment that is used to produce oil or gas from thesereservoirs. Thus, such a sensor system may be provided with a coating asdescribed above, or used as an alternative solution to overcome problemsrecognised by the present inventors that may occur under extremeconditions, as. e.g. may exist in underground oil or gas reservoirs.

With a radial direction is meant a direction from a central point. Incase the cross-section of the core has a circular shape, the centralpoint is the centre of the circle. In case the cross-section of the corehas an elliptical shape, the central point is the centre of the ellipse,i.e. the cross-point of the minor axis of the ellipse with the majoraxis of the ellipse. In case the cross-section of the core has the shapeof a polygon with rotational symmetry, such as a triangle or a square,the central point is the centre of that polygon. With rotationalsymmetry of a polygon is meant that a polygon fits onto itself after aturn of less than 360° around its centre.

A waveguide in a sensor according to the invention usually comprises agrating, in particular a fibre Bragg grating or a long period grating.

In a multiplexed sensor system, the waveguide comprises two or moregratings, e.g. two or more fibre Bragg gratings, two or more long periodgratings, or a combination comprising at least one fibre Bragg gratingand at least one long period grating.

There are different embodiments of a waveguide wherein the thickness ofthe cladding in a first radial direction in the cross-section isdifferent from the thickness of the cladding in a second radialdirection.

In an embodiment, the centre of the circular cross-section of thecladding does not coincide with the centre of the circular cross-sectionof the core (FIG. 2).

In another embodiment, the cross-section of the cladding isnon-circular, e.g. elliptical, while the cross-section of the core iscircular (FIG. 3).

In yet another embodiment, the cross-section of the core isnon-circular, e.g. elliptical, while the cross-section of the claddingitself is circular (FIG. 4).

In yet another embodiment, the cross-section of the core as well as thecross-section of the cladding is non-circular, e.g. elliptical (FIG. 5).

In yet another embodiment, the cross-section of the cladding has anotherdesign than those represented in FIGS. 1-5. Some of such cross-sectiondesigns are shown in FIG. 6. In these designs the coating is present ona specific location of the waveguide where the cladding is thinner orabsent, e.g. in the carves, while locations where the cladding isthicker are not coated or are coated with a coating that has a smallerthickness than the coating on a location of the waveguide where thecladding is thinner or absent.

A specific cross-section design, e.g. a design as shown in any of theFIGS. 2-6, is present along at least a section of the waveguide. Suchsection of the waveguide usually comprises a grating. A specificcross-section design may also extend along a section of the waveguidewhere no gratings are present, and even along the entire waveguide.

Usually, a coating that is capable of deforming under the influence ofthe environmental effect is present on those areas of the cladding wherethe cladding is thinner or absent. Preferably, on those areas of thecladding where the cladding is thicker, such coating is absent or athinner coating of the same kind as that used on a thinner cladding ispresent. It is also possible to provide a different coating on thoseareas of the cladding where the cladding is thicker.

Bragg reflection in a waveguide depends on the optical properties of thecore, the cladding and the coating. The thickness of the claddingdetermines the penetration depth of the electromagnetic radiation in thecoating. If the cladding is thick enough the electromagnetic radiation(evanescent field) will not penetrate into the coating. For example,generally no significant penetration occurs when the cladding has athickness of more than 5 μm, in particular of more than 10 μm. Usually,the thickness of the cladding is about 50 μm when it is desired that nopenetration occurs. A thickness of the cladding of less than 5 μm, inparticular of less than 1 μm, may for example be used to effect asubstantial penetration of electromagnetic radiation into the coating.More in particular, no cladding is present. The diameter of the core isnot critical. and may for instance be in the range of 1-100 μm, inparticular in the range of 5-25 μm, e.g. about 10 μm

The coating is designed to change a dimension (in particular in thelongitudinal direction) and/or the optical properties of the waveguideunder the influence of (a change in) an environmental effect.

In particular, the sensor system comprising a waveguide wherein, in across-sectional plane that is perpendicular to the longitudinaldirection of the waveguide, the thickness of the cladding in a firstradial direction in said plane is different from the thickness of thecladding in a second radial direction in said plane preferably comprisesa source for providing a first beam of polarized electromagneticradiation having the plane of polarization perpendicular to said firstradial direction, and a second beam of polarized electromagneticradiation, having the plane of polarization perpendicular to said secondradial direction.

The angle between the two radial directions is more than 0° and lessthan 180°. In particular, the angle between the directions is in therange of 15°-165°, more in particular the angle is in the range of30°-150°, or in the range of 60°-120°. In practice, it is preferred thatthe directions are perpendicular.

When the plane of polarisation of a beam is perpendicular to the radialdirection wherein the cladding is thicker than the cladding in anotherradial direction, the Bragg-reflection is mainly affected by thedimensions and optical properties of the core-cladding combination. Whenthe plane of polarisation of the other beam is perpendicular to theradial direction wherein the cladding is thinner than the cladding inanother radial direction (or wherein the cladding is absent), and whenon the areas of thinner (or absent) cladding a coating is present, theBragg-reflection is mainly affected by the dimensions and opticalproperties of that core-cladding-coating combination. Thus, it can beachieved that the light of one plane of polarization may be mainlysubject to the dimensions and optical properties of the core-claddingcombination, and the light of the other plane of polarization mainly tothe dimensions and optical properties of the core-cladding-coatingcombination, while the physical period of the grating for both waveguideconfigurations is the same.

In an embodiment, the thickness of the cladding of the waveguide in eachradial direction is equal to the thickness in the radial directionopposite thereto, or at least does not differ significantly from thatdirection. Such embodiments are for example represented by thecross-section designs that are present most left in each of the FIGS.3-6.

In another embodiment the thickness of the cladding in a certain radialdirection is significantly different from the thickness in the radialdirection opposite thereto. Such embodiments are for example representedby the cross-section designs that are present most right in the each ofthe FIGS. 2-5, and by the second cross-section design from the left inFIG. 6. Such cross-sections, i.e. cross-sections wherein the requirementof a thin cladding is met on only one of two opposite sites side mayalso provide the effect described hereinabove, i.e. the effect thatBragg-reflection is mainly affected by the dimensions and opticalproperties of core-cladding-coating combination.

Accordingly, the sensor system, which may be multiplexed, allows thesimultaneous measurement of two environmental effects with one grating,by distinguishing the Bragg reflections in the two polarisationdirections. This is for example advantageous when one wants to correctfor the influence of temperature on the optical properties of thewaveguide when measuring the environmental effect, for example H₂S in anunderground reservoir, or another effect such as mentioned above.Compensation for the influence of the temperature by simply performing acalibration at a specific temperature is generally not sufficient,because temperatures may vary over time and place.

An embodiment with a temperature sensor system as described hereinabovemay advantageously be used under extreme conditions, such as underconditions that may exist in underground oil or gas reservoirs, or inthe equipment that is used to produce oil or gas from these reservoirs.

A suitable coating may be selected from the group of coatings asdescribed hereinabove.

Further, a suitable coating may in particular be selected from materialsas described in the yet to be published European patent applicationEP07150214.0 of which the contents are incorporated by reference, inparticular with respect to details about the coating. An example of amaterial disclosed herein is a material comprising a polymer comprisingan aliphatic chain, which aliphatic chain is provided with functional,preferably hydrophilic, side-chains comprising at least one moietyselected from the group of heterocycloalkyl moieties; in particular, insuch a coating the heterocycloalkyl moiety comprises at least oneheteroatom selected from the group of nitrogen, sulphur and oxygen, andmore in particular, the heterocycloalkyl moiety is selected from thegroup of morpholine moieties, pyrrolidone moieties, pyrrolidinemoieties, oxazolidine moieties, piperidine moieties, tetrahydrofuranmoieties, tetrahydropyran moieties, piperazine moieties and dioxanemoieties. Further details can be obtained from the application of whichthe number is given above.

Further, a suitable coating may in particular be selected from materialsas described in the yet to be published European patent applicationEP07150481.5 of which the contents are incorporated by reference, inparticular with respect to details about the coating. For the coating asensor material may be used comprising a polymer having internal stress,which polymer is capable of at least partially relaxing under theinfluence of the environmental effect; In a specific embodiment, thepolymer having internal stress comprises cross-links, which crosslinksare adapted to be cleaved under the influence of the environmentaleffect; such cross-links may in particular be selected from the group ofamide group cross-links, ester group cross-links, complexed metal ioncross-links, saccharide-based crosslinks, Diels-Alder-based cross-links,diazidostilbene-based cross-links and diperoxide-based cross-links.Further details can be obtained from the application of which the numberis given above.

Further, a suitable coating may in particular comprise a material thatchanges colour and/or refractive index due to (a change in) anenvironmental effect. Such materials may for example be selected fromthe group of transition metal complexes, porphyrines and phtalocyanines.The invention will now be illustrated by the following examples.

EXAMPLE 1

A 25 wt % polycarbonate solution was made by dissolving 20.0 g ofpolycarbonate (PC, Lexan® LR3958) in 60.0 g of dichloromethane. A 25 wt% polysulfone solution was prepared by dissolving 4.0 g of polysulfone(PSU, Ultrason S2010®) in 12.0 g of dimethylacetamide. The solutionswere stirred for 12 hours until clear solutions were obtained. Twoacrylic coated Fibre Bragg Grating (FBG) glass fibres were stripped ofthe acrylic coating. One fibre was dipped twice in the polycarbonatesolution and one fibre was dipped twice in the polysulfone solutionusing a dipcoater to generate a uniform coating and both were cured inan oven at 60° C. The dipcoat speed was 2 mm/s for PSU and 7 mm/s forPC. A coating layer of 50-70 μm was deposited on the fibres. The twofibres were spliced to prepare a single fibre having two sensors. Themaximum wavelength of the reflected light was 1534.85 nm for PC and1529.80 nm for PSU.

The two coated gratings were exposed simultaneously to an increasingconcentration of H₂S in air. Using a FOS&S Spectraleye 600 interogator,the change in reflected maximum wavelength was monitored during theexposure to H₂S gas. The final concentration of H₂S was 90%. Afterexposure the chamber was flushed with dry air in order to remove thehydrogen sulfide gas. The changes in reflected maximum are shown in FIG.7 for both the PSU coated grating and the PC coated grating.

EXAMPLE 2

A series of polysulfone imide sensor polymers was prepared by thereaction of a sulfonyl diamine with a dianhydride. Two diamines and twodianhydrides were used. In Table 1, the molecular formulae of these fourcompounds are shown.

TABLE 1 diamines and dianhydrides for preparation of sensor coatingDiamine I

Diamine II

Dianhydride I

Dianhydride II

One of the diamines was dissolved in N-methyl pyrrolidon (NMP) (20 wt %)and one of the dianhydrides was dissolved in NMP (20 wt %), with theexception of dianhydride I which was dissolved in dimethylsulfoxide(DMSO) (20 wt %). After dissolution, the two solutions were mixed undera nitrogen atmosphere and the polymer solution was obtained. Acryliccoated Fibre Bragg Grating (FBG) glass fibres were stripped of theacrylic coating on the grating over a distance of approximately 20 a 30mm. The stripped fibres were pretreated with a silane coupling agent(y-aminopropyltriethoxysilane) and cured for 180 minutes at 120° C. Thefibres were then dipped in the polysulfone-imide solution to obtain thechemical sensor.

EXAMPLE 3

To create a sensor to detect H₂S gas, a fibre with a fibre Bragg gratingwas coated with a H₂S-sensitive coating. The coating consists of apolysulfoneimide binder formulation with Pebax® as a sensitivecomponent. Pebax® is a polyether block amide and is heat- andUV-resistant. Pebax® swells when placed in contact with H₂S gas. ThePebax® material was added to the coating as particles with a size in therange of 1-5 μm.

0.252 g of a Pebax® elastomer (2533SN01) was mixed with 10.03 g of NMP.Under stirring, the NMP-Pebax® mixture was heated in a glass bottle byan oil bath to 140° C. (the temperature being measured in the NMP/Pebax®mixture), yielding a brown liquid. After melting the Pebax®, 0.25 g ofByk 104S (a surfactant) was added. While still being heated, the mixturewas stirred by an Ultra Turrax set at 20500 min⁻¹ for approximately 5minutes. The bottle with the mixture was removed from the heating bathand stirred again with the Ultra Turrax for a two minutes. After coolingthe bottle to room temperature, it contained a brown dispersion.

The dispersion was visualized by light microscopy (FIG. 2). Theparticles differed in size from approximately 1 to 10 μm. The variationin particle size was decreased by filtration.

The Pebax® dispersion was added to a polysulfone or polyimide solutionprepared according to Example 2, to obtain a blend polymeric materialcomprising nanoparticles.

EXAMPLE 4

To create a sensor to detect CO₂ gas, a fibre with a fibre Bragg gratingwas coated with a CO₂-sensitive polyimide coating. One of the diamines(III-VI) was dissolved in N-methyl pyrrolidone (NMP) (20 wt %). One ofthe anhydrides (III-IV) was added as a solid to the solution (equimolaramount). The mixture was put under a nitrogen atmosphere and mixed atroom temperature for 18 hours. During polymerisation the anhydridedissolved and a clear solution was obtained. Commercial polyimide coatedFibre Bragg Grating (FBG) glass fibres (Optolink) were stripped of thepolyimide coating on the grating over a distance of approximately 20 a30 mm. The stripped fibres were pretreated with a silane coupling agent(γ-aminopropyltriethoxysilane) and cured for 180 minutes at 120° C.Using a Vytran recoater, the new sensor polyimide coating was applied inthin layers, to obtain the chemical sensor.

In Table 2, the molecular formulae of the six compounds are shown.

TABLE 2 diamines and dianhydrides for preparation of CO₂ sensor coatingDiamine III

Diamine IV

Diamine V

Diamine VI

Dianhydride III

Dianhydride IV

1. A sensor system comprising a waveguide, which waveguide comprises agrating in at least a part of the waveguide, which waveguide furthercomprises a coating, the coating comprising a polymer, which polymercomprises a chain, in which chain are present an aromatic group and achemical group selected from the group of sulfonyl groups, carbonylgroups, carbonate groups, siloxane groups, pyridine groups,organofluorine groups and amide groups.
 2. A sensor system according toclaim 1, wherein the chain further comprises imide groups.
 3. A sensorsystem according to claim 1, wherein the chain further comprises oxygenatoms.
 4. A sensor system according to claim 1, wherein the polymer is:a polysulfone, such as a poly (diphenyl sulfone), and preferablypoly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylidene-1,4-phenylene),or poly(4,4′-(sulfonylbis(4,1-phenyleneoxy))dianiline-co-4,4′-(sulfonyl)diphthalic anhydride), or a polycarbonate, such as a poly(diphenylcarbonate), and preferablypoly(oxycarbonyloxy-1,4-phenyleneisopropylidene-1,4-phenylene), or apolyimide, such as an aromatic fluorocarbon polyimide, and preferablypoly(4,4′-(hexafluoro-isopropylidene)dianiline-co-4,4′-(hexafluoro-isopropylidene) diphthalic anhydride) orpoly(2,5 diamino pyridine-co-4,4′-(hexafluoro-isopropylidene) diphthalicanhydride), or a polyamide, preferably poly(trimellitic anhydridechloride-co-4,4′-diaminodiphenylsulfone), or. a polysiloxane, preferablypoly (1,3-bis(3-aminopropyl) tetramethyldisiloxane- co-4,4′-(hexafluoro-isopropylidene) diphthalic anhydride).
 5. A sensorsystem according to claim 1, wherein the grating is a fibre Bragggrating, and wherein the waveguide comprises a plurality of gratingsthat are spatially apart, preferably 2-100 gratings that are spatiallyapart.
 6. A sensor system according to claim 1, wherein the coatingfurther comprises absorbent particles.
 7. A sensor system according toclaim 6, wherein the coating comprises absorbent particles selected fromthe group of elastomeric particles.
 8. A sensor system according toclaim 6, wherein the elastomeric absorbent particles comprise acopolymer of a polyether and a polyamide or an organofluorinecomposition.
 9. A sensor system according to claim 6, wherein thecoating comprises absorbent particles selected from the group of metalorganic framework particles
 10. A sensor system according to claim 1,wherein the polymer coating on the waveguide has a thickness of 0.5 μmto 200 μm, preferably of 10 to 100 μm, more preferably of 20 to 50 μm.11. A sensor system according to claim 1, further comprising a sourcefor providing electromagnetic radiation, and a photo-detector.
 12. Awaveguide, comprising a grating in at least a part of the waveguide,which waveguide comprises a coating, the coating comprising a polymer,which polymer comprises a chain having an aromatic group and a groupselected from the group of sulfonyl groups, carbonyl groups, carbonategroups, siloxane groups, organofluorine groups, pyridine groups andamide groups.
 13. A waveguide according to claim 12, wherein said chainfurther comprises imide groups or oxygen atoms and/or wherein thegrating is a fibre Bragg grating.
 14. A sensor system comprising awaveguide, which waveguide comprises a core and a cladding at leastpartially covering the core, wherein, in a cross-sectional plane that isperpendicular to the longitudinal direction of the waveguide, thethickness of the cladding in a first radial direction in said plane isdifferent from the thickness of the cladding in a second radialdirection in said plane, the waveguide further comprising a coatingsensitive to a change in an environmental effect, which coatingcomprises a polymer, which polymer comprises a chain, in which chain arepresent an aromatic group and a chemical group selected from the groupof sulfonyl groups, carbonyl groups, carbonate groups, siloxane groups,pyridine groups, organofluorine groups and amide groups or a differentcoating sensitive to a change in an environmental effect.
 15. A sensorsystem according to claim 14, comprising a source for providing a firstbeam of polarized electromagnetic radiation having a plane ofpolarization perpendicular to said first radial direction, and a secondbeam of polarized electromagnetic radiation, having a plane ofpolarization perpendicular to said second radial direction.
 16. Use of asensor system according to claim 1 for detecting in an environment atleast one analyte selected from the group of alkanes (e.g. methane,ethane, propane), carbon dioxide, hydrogen sulphide, hydrogen, water,carbon monoxide, oxygen, hydrogen cyanide and ammonia.
 17. Use of asensor system according to claim 1 for the detection of one or moreenvironmental effects on two or more spatially distinct sites, whereinthe waveguide comprises a multitude of gratings, at least part of whichare present as pairs, in which pairs the first grating is used tomeasure a certain environmental effect, and the second grating isuncoated or is coated with a coating that is insensitive, or at leastnot sensitive to a measurable extent, to the environmental effect to bemeasured with the first grating.
 18. Use of a sensor system according toclaim 1 for measuring an environmental effect under non-ambientconditions, in particular under conditions that may exist in undergroundoil or gas reservoirs, or in the equipment that is used to produce oilor gas from these reservoirs.